I can't understand your diagram, too.
To clarify, I was asking if falling water from a brook (which is the same level as my well) and becoming a waterfall pressurize itself to the same level as its source even when disconnected from it?
How can water flow from the brook to your well if the path of water is disconnected?
Ok, I realize my diagram is broken. The well spans z-levels right? Lets just ignore my diagram and say that water flows OFF the edge of the brook and becomes a waterfall falling into a brook below, continuing it. I plan to bar it halfway and redirect the flow of the falling water. My question was that will the water pressurize and go up out of the well into my fortress as the source (first brook) is at the same z-level?
I'm not quite following what you're saying...but there might be a way to do what you want. In order to have water flowing in, you must also have water flowing out, or you'll flood, obviously. You can use depressurization to stop the flow, if you connect the river directly to your cistern, without a waterfall, but with a continuously flowing waterfall, you must always have a drain.
If your cistern has a path to the edge of the map, and you make that path wider than your inlet (plus some extra if you're running through fortifications on the bottom but not on the top), it will be possible to have a waterfall fill your cistern without flooding your base. It'd be something like this, viewed from the side:
+ is a wall
blue stuff is water
_ is floor
W is a well
++++++++++~77777
++++++++++~+++++
++++++++++~+++++
++++++++++~+++++
__________W____+
333333333333333+
++++++777777777+
++++++777777777+
++++++++++++++++
Note that you get something like 3s in the drain channel because you have a much wider drain channel than your inlet channel, with lots of fortifications at the edge of the map to let the water flow out, so you have more outflow capacity than water flowing in.
If you were planning to use a brook at a lower level as the drain, instead of fortifications off the edge of the map, I'm not sure whether it'd work or not.
Can someone confirm that this waterfall design would work? Or does it need any improvements for drainage or mist generation? I haven't built one myself, so while I know the cistern wouldn't flood, I'm not sure whether water would land outside the well and flood the fortress that way if some floor gratings aren't built near the well for additional drainage, whether a waterfall directly into a well is a good idea or not in the first place, and whether mist will be generated by this design or not.